I too have been on the hunt for one, but they are scarce. 

One source is the Rifle Shoppe. They sell castings for muzzle loading rifles, but do have some castings for the second model Maynard, and that buttplate was used on all Maynards through the end of production, excepting the Nos. 14, 15 and 16. 

Other sources for Maynard second model parts are Lodgewood and S&S Firearms. 

Good luck!